1. Field of the Invention
The present invention relates to photographing apparatus, and more particularly to a coaxial master-slave lens photographing apparatus.
2. Description of Prior Art
Conventional photographing equipment includes two major categories: the camera and the camcorder. The camera is further divided into two types: the single lens reflection (SLR) type and the lens shutter type. For any category or type of photographing apparatus, three optical systems must be included:
(1) photographing optical system; PA1 (2) viewfinder optical system; and PA1 (3) focusing optical system.
(1) Photographing optical system
In general, the photographing optical system is comprised of a master lens and a photo-sensing device. FIG.1 shows the structure of a conventional SLR type camera 50. To photograph an object, a light ray 10 reflected by the object passes through master lens group 1 and then forms an image on film 8.
(2) Viewfinder optical system
The structure of the viewfinder optical system of the conventional SLR type camera 50 is also shown in FIG.1. Light ray 10 passes through the master lens group 1 and then reaches reflection mirror 3 to be reflected to pentagon prism 4. Reflected twice within the pentagon prism 4, the light ray 10 reaches on eyepiece 2 and multi-sensor photo-detector 5. Through the eyepiece 2, the camera user may thus see the image of the object. FIGS. 2A-2B show the SLR type camera when used respectively in viewfinding and photographing; FIGS. 3A-3C show the SLR type camera when the mirror raises and the shutter is open during viewfinding.
From FIGS. 2A-2B, it can be seen that to photograph the image of the object, the reflection mirror 3 should be raised so as to allow light from the main lens group 1 to pass to the film 8. For this reason, viewfinding and photographing can not be carried out at the same time. Still, as shown in FIG.3, after the shutter release button is pressed for photographing, the shutter can be released only after the reflection mirror 3 has been raised. Opening the shutter thus can not be carried out directly after the shutter release button is pressed. Moreover to these disadvantages, it can be seen from FIG. 1 that the conventional SLR type camera is very complicated in the structure of its viewfinder optical system.
FIG. 4 shows the structure of a conventional shutter-in-lens type camera. In this type of camera, the viewfinder optical system and the photographing optical system are independent to each other: light ray 10a passes lens 2a and then reaches eyepiece 2. As illustrated in FIG. 5, the optical axis 30 of the viewfinding optical system is not in coincidence with the optical axis 20 of the photographing optical system. As a consequence, the imaging of object in the viewfinder optical system is not the same as that in the photographing optical system. Also, as shown in FIG. 6, when a zoom lens is used, the lens group 12 of this type of camera should be focus-varied corresponding to the variation of focus in the zoom lens. The viewfinder optical system is thus very complicated in structure.
FIG. 7 shows the structure of a conventional camcorder. A light ray 10 passes through first lens group 1a and then reaches photo-splitter 15 to be divided into two rays, first ray 10b and second ray 10c. The first ray 10b, after passing through second lens group 1b, forms an image on image plane 8; while the second ray 10c, after passing through third lens group 1c, forms an image on CCD photo-sensor array 1d. The image signal generated by the CCD photo-sensor array is subsequently processed by image processor 1e to form the photographed images on CRT display 1f (LCD display can be used instead). The displayed images can be viewed by the user through eyepiece lens group 2. This type of camcorder as described has three drawbacks. First, the camcorder has a complex structure that requires a great number of components for construction; and CRT, which is expensive, is included in the structure, so that cost is too high to be affordable by consumers. Second, with the provision of the photo-splitter, the light ray 10b is reduced in intensity before forming the image. Third, the image processor and the CRT are components that consume a great amount of electric power.
The following table summarizes and compares the advantages and disadvantages of viewfinder optical systems respectively in the conventional SLR type camera, shutter-in-lens camera, and camcorder.
TABLE 1 __________________________________________________________________________ Viewfinder Optical Systems SLR Shutter-in- camera lens camera camcorder __________________________________________________________________________ Design optical axis no in Independent of optical axis not in Principle coincidence with photographing coincidence with that of photograph- system that of photograph- ing optical system ing optical system using a jumping using a fixed light plane mirror splitter (see FIG. 1) Performance Good Poor Good Poor Good Poor Viewfinder Yes No Yes optical axis (see in coin- FIG. 5) cidence with that of photographing optical system? Light flux Yes Yes No not split by (see viewfinder FIG. 7) system and thus reduced in intensity? Photographing No Yes Yes and viewfind- (see ing able to FIG. 3) function concurrently? Shutter No Yes Yes opened right (see after the FIG. 3) shutter release button is pressed? Viewfinder No Yes Yes system more (see light and FIG. 1) compact when fixed-focus lens is mounted? Viewfinder No No Yes system more (see (see light and FIG. 1) FIG. 1) compact when zoom lens is mounted? Viewfinding Yes Yes No and imaging require power consumption? Resolution of Yes Yes No the image in viewfinder high? Image in Yes Yes No viewfinder (see close to FIG. 7) true color? __________________________________________________________________________
(3) Focusing optical system
FIG. 8A shows the structure of focusing optical system in SLR type camera. Light entering into the lens passes in part through semi-reflection mirror 3 and then, after reflected by full-reflection mirror 3a, reaches at focusing error correcting unit which is coupled to focus adjustment control motor 6. Working principle of the focusing error correction is illustrated in FIG. 8B, which is a conventional technique so that details will not be described.
The conventional shutter-in-lens type camera is constructed based on either of two principles, respectively illustrated in FIGS. 9A-9B and FIG. 10. In the system of FIG. 9A, light rays reflected from an object enter the focusing optical system through two apertures 41 and 42, thus forming two separate images as shown in FIG. 9B. Focusing is effected based on the intensity difference between the two images. Drawbacks of this system include that the optical axis of the focusing optical system is not in coincidence with that of the photographing optical system; the focusing optical system and the viewfinder optical system can not monitor each other; and the mechanical construction for the focusing optical system is very complicated. On the other hand, in the system of FIG. 10, a photo-emitting device 16 sends out a light beam to an object being photographed and a photo-sensor 17 receives the reflection of the light beam from the object for focusing. As illustrated in FIG. 11, drawbacks of this system include that the system may mistake object AA', which is to be focused, for another object BB'; and that the focusing mechanism and the viewfinder can not monitor each other.
The following table summarizes and compares the advantages and disadvantages of focusing optical systems respectively in the conventional SLR type camera, shutter-in-lens camera, and camcorder.
TABLE 2 __________________________________________________________________________ Focusing optical System SLR Shutter-in-lens camera camera camcorder __________________________________________________________________________ Illumina- Same as Same as Reflect a Same as tion of photograph- photograph- light beam photo- object ing system ing system from the graphing camera system Method of Use part of Independent Independent Completely collecting photograph- of photo- of photo- dependent object system and graphing graphing on the light then split system; system; photo- the light use two use one graphing with the separate single system jumping apertures aperture mirror for imaging for imaging Processing Split the Overlap Use photo- Perform of object object the split imparting FFT analy- light image and images for sensors sis to the the compare comparison object the two. image Reference FIG. 8 FIG. 9 FIG. 10 FIG. 12 FIG. Performance Good Poor Good Poor Good Poor Good Poor Optical Yes No Yes Yes axis in (see coincidence FIG. 9) with that of the photograph- ing system? Not mistake Yes Yes No No other (see (see objects for FIG. 11) FIG. 11) the one being under focusing? Light source Yes Yes No Yes need not to be self- prepared? Processing Yes Yes Yes No of electric signal generated by the focusing system not complex? Focusing Yes No No Yes and view- finding able to monitor each other? Mechanical No No Yes Yes construct- (see (see ion of the FIG. 8) FIG. 9) focusing system not complex, i.e. does not have to actuate a plane mirror? __________________________________________________________________________